Not very clear. Let me summarize it. You have:
------- LAN ------------ WAN ------------
| IIS |---------| Firewall |---------| Computer |
------- ------------ ------------
Is this correct?
And you say, from Computer you can access IIS with RDP, but not it's website?
The reason should be that, on the firewall, there is a NAT (Network Address Translation) rule that redirects incoming packets on port 3389 (rdp) to IIS Server.
If you want to also access the website, then there should be a rule for port 80 (http) and/or 443 (https).
EDIT:
It could also be possible that your website is listening on address 127.0.0.1 (localhost), but not on LAN address (which I don't know). You can fix that in IIS MMC console. Not it can't, I misread and thought the site were only accesible through localhost.